Charge-transfer Satellites Research Articles

Negative charge-transfer energy in SrFeO3 revisited with hard x-ray photoemission spectroscopy

We report hard x-ray photoelectron spectroscopy on SrFeO3 which is one of the classical conducting transition-metal oxides with a noncollinear magnetic structure. The obtained spectra show a detailed charge-transfer (CT) satellite structure, the Fe 2p3/2 main peak exhibits multiplet splitting, and the deterioration signs present in previous reports are absent here, allowing for a better determination of its intrinsic electronic structure. The results are well described by a FeO6 cluster model with a charge-transfer energy of about −1.0 eV, confirming the values obtained in the previous works. The negative CT energy indicates that the electronic configuration of the tetravalent Fe is d5L rather than d4 where L represents an O 2p hole. The small spectral weight observed at the Fermi level indicates the correlated metallic state with localized Fe 3d electrons and mobile O 2p holes which are governed by a large d−d Coulomb interaction and negative CT energy. Published by the American Physical Society 2024

Bulk and surface electronic structure of MnPSe3 revealed by photoemission and x-ray absorption spectroscopy

Bulk and surface electronic states of ${\mathrm{MnPSe}}_{3}$ have been investigated by means of x-ray photoemission spectroscopy (XPS), x-ray absorption spectroscopy (XAS), and theoretical calculations. In Mn $2p$ XPS, the main peak is accompanied by a charge-transfer satellite which is commonly observed in various Mn chalcogenides. The intensity of the charge-transfer satellite is considerably reduced by oxidization of the surface. The multiplet structure of Mn $2p$ XAS indicates the high spin ${\mathrm{Mn}}^{2+}$ configuration. The cluster model analyses of the XPS and XAS spectra indicate that the Se $4p$-to-Mn $3d$ charge-transfer energy $\mathrm{\ensuremath{\Delta}}$ is negative providing the strong $p\ensuremath{-}d$ hybridization. The magnetic interaction between the third nearest neighboring Mn spins is enhanced by the smallness of $\mathrm{\ensuremath{\Delta}}$ and the unique molecular orbitals of the ${\mathrm{P}}_{2}{\mathrm{Se}}_{6}$ cluster.

Reconstruction of pyrrhotite fracture surfaces

The interpretation of core level Synchrotron photoemisison spectra (SPES) collected from a pristine fracture surface of monoclinic pyrrhotite (Fe7S8), as a function of photon energy, has identified the three distinct surface species. The Fe 2p photoemission spectra show the Fe in pyrrhotite is high spin FeII in octahedral co-ordination. Comparison of bulk and surface sensitive spectra did not identify any changes due to the broad nature of the high spin Fe 2p spectra. However, the sulfur spectrum shows an undercoordinated monosulfide at 160.8 eV, a disulfide (161.7 eV) and polysulfide species (163.2 eV) at the surface. This indicates stabilisation of the surface involves reconstruction and results in chemical transformation of sulfur species. A high binding energy tail that extends to 167 eV has been attributed to ligand to metal charge transfer satellites (LMCT) and supported by the pre-edge features observed in Fe L2,3 and S L2,3 near edge X-ray absorption fine structure (NEXAFS). An accurate fit for the bulk pyrrhotite sulfide spectrum has been estimated by comparing spectra with different surface sensitivities, and thereby an accurate fitting method has been developed for the monoclinic pyrrhotite S 2p spectrum. This method may be used for future photoemission spectra to improve the analysis of similar samples and provide a basis for interpretation of pyrrhotite samples during mineral processing and environmental conditions.

AbInitio Multiplet-Plus-Cumulant Approach for Correlation Effects in X-Ray Photoelectron Spectroscopy.

The treatment of electronic correlations in open-shell systems is among the most challenging problems of condensed matter theory. Current approximations are only partly successful. Ligand-field multiplet theory has been widely successful in describing intra-atomic correlation effects in x-ray spectra, but typically ignores itinerant states. The cumulant expansion for the one-electron Green's function has been successful in describing shake-up effects but ignores atomic multiplets. More complete methods, such as dynamic mean-field theory can be computationally demanding. Here, we show that separating the dynamic Coulomb interactions into local and longer-range parts with abinitio parameters yields a combined multiplet-plus-cumulant approach that accounts for both local atomic multiplets and satellite excitations. The approach is illustrated in transition metal oxides and explains the multiplet peaks, charge-transfer satellites, and distributed background features observed in XPS experiment.

Charge correlation in V2OPO4 probed by hard x-ray photoemission spectroscopy

Electronic properties of V$_2$OPO$_4$ have been investigated by means of hard x-ray photoemission spectroscopy (HAXPES) and subsequent theoretical calculations. The V 1$s$ and 2$p$ HAXPES spectra are consistent with the charge ordering of V$^{2+}$ and V$^{3+}$. The binding energy difference between the V$^{2+}$ and V$^{3+}$ components is unexpectedly large indicating large bonding-antibonding splitting between them in the final states of core level photoemission. The V 1$s$ HAXPES spectrum exhibits a charge transfer satellite which can be analyzed by configuration interaction calculations on a V$_2$O$_9$ cluster. The V 3$d$ spectral weight near the Fermi level is assigned to the 3$d$ $t_{2g}$ orbitals of the V$^{2+}$ site. The broad V 3$d$ spectral distribution is consistent with the strong hybridization between V$^{2+}$ and V$^{3+}$ in the ground state. The core level and valence band HAXPES results indicate substantial charge transfer from the V$^{2+}$ site to the V$^{3+}$ site.7 figure

New interpretation and approach to curve fitting synchrotron X-ray photoelectron spectra of (Fe,Ni)9S8 fracture surfaces

The surface electronic structure of (Fe,Ni)9S8 was investigated using synchrotron X-ray photoelectron spectroscopy (SXPS). SXPS S 2p and valence band (VB) spectra, collected as a function of photon energy, have resolved two distinct bulk states and two surface chemical states present at (Fe,Ni)9S8 fracture surfaces. The SXPS S 2p spectra show two distinct sulfur sites within the lattice that have been attributed to 4-coordinate sulfur at 161.7 eV, and 5-coordinate sulfur at 162.2 eV. Upon fracture, sulfur sites are exposed at multiple fracture planes and lose at least one bonding partner, resulting in surface 3-coordinate sulfur at 161.1 eV, and a slightly broader peak at 161.7 eV attributed to 4-coordinate sulfur surface monomer species overlapping the bulk 4-coordinate species. Significant asymmetric tail shapes were observed in each of the S 2p spectra and have been identified as ligand-to-metal charge transfer satellites (LMCT). A practical approach for curve fitting the (Fe,Ni)9S8 XPS spectra has been proposed.

Spectral Change in 3d–4f Resonant Inelastic X-ray Scattering of Ce Intermetallics Across the Transition between Kondo Singlet and Localized-Spin State

The spectral change in the 3d resonant X-ray inelastic scattering (RIXS) induced by the spin-state transition between Kondo singlet (KS) and localized spin (LS) state is theoretically investigated for γ-like Ce intermetallics by means of a single impurity Anderson model. The basis configurations with an electron–hole pair are included in the calculation within the configuration interaction scheme, in addition to the intra-atomic full multiplet coupling of the Ce impurity. A distinct spectral change is found across the KS–LS transition in the RIXS excited at the charge-transfer satellite of the 3d X-ray absorption spectrum (XAS) under a polarized geometry. In contrast, the 3d XAS and RIXS spectra under a depolarized geometry are rather insensitive to the spin-state transition.

Electronic structure of single-crystal solid solutions Pb1-xBaxTiO3 (0 ≤ x ≤ 1) from X-ray photoelectron spectroscopy and real-space multiple electron scattering calculations

Valence band and core level X-ray photoelectron spectra (XPS) are measured in single-crystal solid solutions Pb1-xBaxTiO3 (x = 0, 0.021, 0.146, 0.24, 0.541, 0.784, 0.906, 1). Valence band densities of states (DOS) and theoretical valence band XPS are calculated for Pb1-xBaxTiO3 (x = 0, 0.12, 0.25, 0.5, 0.875, 1) via the method of multiple electron scattering in real space. Experimental and theoretical valence band XPS are in good agreement. The increase of ionicity of the TiO bonds in Pb1-xBaxTiO3 upon the growth of relative Ba content x is predicted theoretically and shown experimentally using the energy position of the charge transfer satellites in the Ti2p XPS. The increase of Ti2p-spectrum line widths against relative Ba content x is observed.

High-resolution X-ray absorption spectroscopy as a probe of crystal-field and covalency effects in actinide compounds

Applying the high-energy resolution fluorescence-detection (HERFD) mode of X-ray absorption spectroscopy (XAS), we were able to probe, for the first time to our knowledge, the crystalline electric field (CEF) splittings of the [Formula: see text] shell directly in the HERFD-XAS spectra of actinides. Using ThO2 as an example, data measured at the Th 3d edge were interpreted within the framework of the Anderson impurity model. Because the charge-transfer satellites were also resolved in the HERFD-XAS spectra, the analysis of these satellites revealed that ThO2 is not an ionic compound as previously believed. The Th [Formula: see text] occupancy in the ground state was estimated to be twice that of the Th [Formula: see text] states. We demonstrate that HERFD-XAS allows for characterization of the CEF interaction and degree of covalency in the ground state of actinide compounds as it is extensively done for 3d transition metal systems.

Crystal‐Field and Covalency Effects in Uranates: An X‐ray Spectroscopic Study

The electronic structure of U(V) - and U(VI) -containing uranates NaUO3 and Pb3 UO6 was studied by using an advanced technique, namely X-ray absorption spectroscopy (XAS) in high-energy-resolution fluorescence-detection (HERFD) mode. Due to a significant reduction in core-hole lifetime broadening, the crystal-field splittings of the 5f shell were probed directly in HERFD-XAS spectra collected at the U 3d edge, which is not possible by using conventional XAS. In addition, the charge-transfer satellites that result from U 5f-O 2p hybridization were clearly resolved. The crystal-field parameters, 5f occupancy, and degree of covalency of the chemical bonding in these uranates were estimated by using the Anderson impurity model by calculating the U 3d HERFD-XAS, conventional XAS, core-to-core (U 4f-3d transitions) resonant inelastic X-ray scattering (RIXS), and U 4f X-ray photoelectron spectra. The crystal field was found to be strong in these systems and the 5f occupancy was determined to be 1.32 and 0.84 electrons in the ground state for NaUO3 and Pb3 UO6 , respectively, which indicates a significant covalent character for these compounds.

Cation diffusion and hybridization effects at the Mn-GaSe(0001) reacted interface:Ab initiocalculations and soft x-ray electron spectroscopy studies

The electronic properties of the Mn:GaSe interface, produced by evaporating Mn at room temperature on a $\ensuremath{\epsilon}$-GaSe(0001) single-crystal surface, have been studied by soft x-ray spectroscopies, and the experimental results are discussed at the light of ab initio DFT+U calculations of a model ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Se} (x=0.055)$ surface alloy. Consistently with these calculations that also predict a high magnetic moment for the Mn ions $(4.73\text{--}4.83 {\ensuremath{\mu}}_{B})$, XAS measurements at the Mn $L$ edge indicate that Mn diffuses into the lattice as a ${\mathrm{Mn}}^{2+}$ cation with negligible crystal-field effects. Ab initio calculations also show that the most energetically favorable lattice sites for Mn diffusion are those where Mn substitutes Ga cations in the Ga layers of the topmost Se-Ga-Ga-Se sandwich. Mn $s$ and $p$ states are found to strongly hybridize with Se and Ga $p$ states, while weaker hybridization is predicted for Mn $d$ states with Se $s$ and $p$ orbitals. Furthermore, unlike other Mn-doped semiconductors, there is strong interaction between the $\mathrm{Ga}\text{\ensuremath{-}}s$ and $\mathrm{Mn}\text{\ensuremath{-}}{d}_{{z}^{2}}$ states. The effects of hybridization of Mn $3d$ electrons with neighboring atoms are still clearly detectable from the characteristic charge-transfer satellites observed in the photoemission spectra. The Mn $3d$ spectral weight in the valence band is probed by resonant photoemission spectroscopy at the Mn $L$ edge, which also allowed an estimation of the charge transfer ($\mathrm{\ensuremath{\Delta}}=2.95$ eV) and Mott-Hubbard ($U=6.4$ eV) energies on the basis of impurity-cluster configuration-interaction model of the photoemission process. The Mott-Hubbard correlation energy $U$ is consistent with the ${U}_{\mathrm{eff}}$ on-site Coulomb repulsion parameter (5.84 eV) determined for the ab initio calculations.

Bi1-xCaxFeO3-δ (0 ≤ x ≤ 1) ceramics: Crystal structure, phase and elemental composition, and chemical bonding from X-ray diffraction, Raman scattering, Mössbauer, and X-ray photoelectron spectra

An integrated study of samples of Bi1−хCaхFeO3-δ (BCFO) system for 0 ≤ x ≤ 1 (step 0.1) is performed using X-ray diffraction (XRD), Raman scattering, Mössbauer spectroscopy, and X-ray photoelectron spectroscopy (XPS). Measured elemental compositions of samples were found to deviate substantially from nominal ones. X-ray structure analysis discovered a continuous series of solid solutions with perovskite structure (or its derivatives) on the interval of nominal Ca content 0 ≤ x ≤ 0.9. However, the analysis of the data from all the methods used, limits the region of single phase samples (in presence of insignificant amount of foreign ones) to nominal Ca content interval 0.1 < x < 0.4. No Fe4+ ions are discovered upon substitution of Bi3+ by Ca2+, neither by Mössbauer spectroscopy, nor by XPS. Charge compensation in Bi1−хCaхFeO3-δ takes place mostly via creation of oxygen vacancies. Charge transfer satellites position in Ca2p XPS is shown to be sensitive to a number of oxygen ions in the immediate environment of Ca.

Effects of Nd in Nd&lt;sub&gt;&lt;italic&gt;x&lt;/italic&gt;&lt;/sub&gt;In&lt;sub&gt;1-&lt;italic&gt;x&lt;/italic&gt;&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Semiconductors for Thin-Film Transistors

Thin-film transistors (TFTs) with neodymium-substituted indium oxide [Nd x In1- x O3 (NIO)] semiconductor were fabricated. It was found that NIO films with higher annealing temperature have higher film quality and higher mobility; and NIO films with higher Nd concentration have higher coordination number of In, indicating fewer oxygen vacancies and lower free carrier density. TFTs with different Nd concentration exhibited good electrical stability under positive gate-bias stress, but the ones with lower Nd concentration (5%) displayed much poorer stability under negative gate-bias stress (NBS) compared with those with higher Nd concentration (15% and 25%). Detailed studies showed that the charge transfer satellite with the electron configuration of $\vert $ Nd $3{\rm d}_{5/2}^{5} 4f^{4}\text{O}2p^{-1}\rangle $ , which originates from the $\textrm {O}2p \to \textrm {Nd}4f$ charge transfer process, was the main reason for poor NBS stability for TFTs with 5% Nd, because it favored a $4f^{n+l}/4f^{n}$ couple formation, resulting in the formation of donor/acceptor states, which will affect the electrical stability.

Real-time cumulant approach for charge-transfer satellites in x-ray photoemission spectra

X-ray photoemission spectra generally exhibit satellite features beyond the main peak due to many-body excitations. However, the satellites associated with charge-transfer excitations in correlated materials have proved difficult to calculate from first principles and their interpretation has been controversial. Here we show that these satellites can be attributed to local density fluctuations in response to a suddenly created core hole. Our approach is based on a cumulant representation of the core-hole Green's function with a real-time, time-dependent density functional theory calculation of the cumulant. This approach includes effects that cannot be accounted for by cluster methods and yields a direct real-space, real-time interpretation. Illustrative results for ${\mathrm{TiO}}_{2}$ and NiO are in good agreement with XPS experiment.

Coexistence of localized and itinerant electrons inBaFe2X3(X=Sand Se) revealed by photoemission spectroscopy

We report a photoemission study at room temperature on BaFe2X3 (X = S and Se) and CsFe2Se3 in which two-leg ladders are formed by the Fe sites. The Fe 2p core-level peaks of BaFe2X3 are broad and exhibit two components, indicating that itinerant and localized Fe 3d sites coexist similar to KxFe2-ySe2. The Fe 2p core-level peak of CsFe2Se3 is rather sharp and is accompanied by a charge-transfer satellite. The insulating ground state of CsFe2Se3 can be viewed as a Fe2+ Mott insulator in spite of the formal valence of +2.5. The itinerant versus localized behaviors can be associated with the stability of chalcogen p holes in the two-leg ladder structure.

Charge transfer satellites in x-ray spectra of transition metal oxides

Strongly correlated materials such as transition metal oxides (TMOs) often exhibit large satellites in their x-ray photoemission (XPS) and x-ray absorption spectra (XAS). These satellites arise from localized charge-transfer (CT) excitations that accompany the sudden creation of a core hole. Here we use a two-step approach to treat such excitations in a localized system embedded in a condensed system and coupled to a photoelectron. The total XAS is then given by a convolution of a spectral function representing the localized excitations and the XAS of the extended system. The local system is modeled roughly in terms of a simple three-level model, leading to a double-pole approximation for the spectral function that represents dynamically weighted contributions from the dominant neutral and charge-transfer excitations. This method is implemented using a resolvent approach, with potentials, radial wave-functions and matrix elements from the real-space Green's function code FEFF, and parameters fitted to XPS experiments. Representative calculations for several TMOs are found to be in reasonable agreement with experiment.

Electronic structure of the hole-doped delafossite oxides CuCr1−xMgxO2

We report the detailed electronic structure of a hole-doped delafossite oxide CuCr${}_{1\ensuremath{-}x}$Mg${}_{x}$O${}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}0.03$) studied by photoemission spectroscopy (PES), soft x-ray absorption spectroscopy (XAS), and band-structure calculations within the local-density approximation $+U$ ($\mathrm{LDA}+U$) scheme. Cr/Cu 3$p$-$3d$ resonant PES reveals that the near-Fermi-level leading structure has primarily the Cr $3d$ character with a minor contribution from the Cu $3d$ through Cu $3d\ensuremath{-}$O $2p\ensuremath{-}$Cr $3d$ hybridization, having good agreement with the band-structure calculations. This indicates that a doped hole will have primarily the Cr $3d$ character. Cr $2p$ PES and $L$-edge XAS spectra exhibit typical Cr${}^{3+}$ features for all $x$, while the Cu $L$-edge XAS spectra exhibited a systematic change with $x$. This indicates now that the Cu valence is monovalent at $x=0$ and the doped hole should have Cu $3d$ character. Nevertheless, we surprisingly observed two types of charge-transfer satellites that should be attributed to Cu${}^{+}$ ($3{d}^{10}$) and Cu${}^{2+}$ ($3{d}^{9}$) like initial states in Cu $2p$-$3d$ resonant PES spectrum of at $x=0$, while Cu $2p$ PES spectra with no doubt shows the Cu${}^{+}$ character even for the lightly doped samples. We propose that these contradictory results can be understood by introducing not only the Cu $4s$ state, but also finite Cu $3d,4s\ensuremath{-}$Cr $3d$ charge transfer via O $2p$ states in the ground-state electronic configuration.

X-ray photoelectron spectroscopy study of electron and spatial structure of mono- and binuclear Ni(II) carboxylate complexes with nitrogen-containing ligands

Ni3 s, Ni3 p and Ni2 p X-ray photoelectron spectra of mono- and binuclear carboxylate complexes of nickel with various geometry of metal atoms environment are obtained. The spectra are calculated in an isolated ion approximation. The dependence of the spectral profiles and the structure of the charge-transfer satellites on the structure of the immediate environment of nickel atoms is established. The data obtained support the results of X-ray diffraction and magnetic studies.

Transport and x-ray photoelectron spectroscopy properties of (Ni1−xCux)Mn2O4 and Ni(Mn2−yCuy)O4

The temperature dependence of the electrical resistivity and the thermoelectric power in (Ni1−xCux)Mn2O4 (x = 0 to 0.5) and Ni(Mn2−yCuy)O4 (y = 0 to 0.2) are investigated. The electrical resistivity of samples with x &amp;lt; 0.1 appears to be hardly changed with increasing Cu content. In all of the samples, the electron transfer is thermally activated and shows semiconducting behavior. The electric conductivity is described by a small polaron hopping mechanism except for x ≥ 0.3. The thermoelectric power at 100 °C is found to change sign from negative to positive with Cu substitution both in (Ni1−xCux)Mn2O4 and Ni(Mn2−yCuy)O4. Furthermore, the thermoelectric powers of these samples with x, y &amp;lt; 0.1 are found to change sign from negative to positive with increasing temperature. The valence distribution of the Mn ions is estimated using x-ray photoelectron spectroscopy. The peak intensity ratio of Mn3+/Mn4+ is maximized whereas that of Ni3+/Ni2+ is minimized at about x, y = 0.05 to 0.07. These results suggest that the Mn4+ and Ni2+ ions change disproportionately into Mn3+ and Ni3+ ions with increasing Cu content up to x, y = 0.1. The valence states of Ni in the system are in accordance with those of Mn, which is necessary in order to maintain charge neutrality and oxygen stoichiometry. The normalized peak intensity of the charge transfer satellite peak of Mn 2p3/2 is rapidly increased by Cu substitution up to x, y = 0.1. With further Cu substitution when x, y &amp;gt; 0.1, the ratio of increase in the peak intensity of the charge transfer satellite is decreased. These facts suggest that the decrease of electrical resistivity when x, y ≥ 0.1 is caused by an increase of holes having O 2p character, and the changes of sign in the thermoelectric power might take care of the competition between the electron conduction term and the hole conduction term caused by the charge transfer from O 2p to transition metal 3d orbitals.

Electric and thermoelectric properties in Cu1+xMn2-xO4+δ

The temperature dependencies of the electrical conductivity and the thermoelectricpower in Cu1+xMn2-xO4+δ (x = 0 – 0.5) are investigated. The electron transfer is thermally activated and displays a semiconducting behavior. The thermoelectric power shows p-type conduction in all temperature range and decreases with increasing temperature in all samples.The thermoelectric power factor shows a maximum value for x = 0.4 at 600 °C. According to x-ray photoelectron spectroscopy the charge transfer satellites are observed in energy spectrumabout 6 eV from the main peak in all samples. The existence of the charge transfer satellites indicates that there are strong Coulomb interactions between the 3d electrons of the transitionmetal and strong hybridization between the transition metal 3d and the ligand O 2p orbitals.However, the peak intensity of charge transfer of Mn 2p3/2 is hardly affected by Cu substitution.This result suggests that the hybridization of the Mn 3d and O 2p states hardly contribute to the change in the transport properties by Cu substitution.